Conventional structural assemblies, such as those used in the manufacture of military and commercial aircraft, are commonly fabricated from a plurality of structural members secured together to form a built-up structure. The structural members are typically forged, machined from stock material or cast in various configurations from steel, stainless steel, magnesium, magnesium alloys, copper, copper alloys, brass, aluminum, aluminum alloys, or titanium.
During use, aircraft structural assemblies are subjected to static and cyclic loads, as well as a variety of environmental conditions, temperature variations, and severe acoustic and vibration environments, all of which create mechanical and thermal stresses. While these operational stresses generally exist throughout the individual structural members forming the structural assembly, certain regions of each structural member are typically subjected to comparatively higher magnitudes of stress. For example, under cyclic loading conditions, threaded openings machined into a structural member to facilitate attachment to other structural members when forming a structural assembly can significantly increase the stress in the immediate vicinity of the opening. High operational stresses can lead to micro-cracking or fracture of the structural members of a structural assembly, which can result in the eventual failure of the assembly. In addition, due to the large number of parts and fasteners utilized in the construction of conventional structural assemblies, maintenance, repair and replacement of structural members, if necessary, can be time consuming and labor intensive, which can be costly over the life of the assembly.
In seeking to enhance the strength, toughness and fatigue resistance of structural members and, thus, increase the useful life of structural assemblies, designers have modified the dimensions of the structural members in the regions of high operational stress, for example, by increasing the thickness of the members in these regions. Designers have also experimented with substituting more exotic and, typically, more expensive types of materials for use in the fabrication of the structural members. Structural members can also undergo precipitation hardening whereby the members are solution heat treated and then aged at predetermined temperature schedules to thereby improve the grain structure and, thus, the material properties of the members. However, the precipitation hardening process can be time and labor intensive and provides only limited improvement of material properties, and even selective increases in the thickness of a structural member can negatively increase the overall weight of the structural assembly, as well as resulting in increased material cost.
Accordingly, there remains a need for improved structural members and methods of manufacture that will increase the operational life of structural assemblies. The improved structural members must have enhanced strength, toughness and fatigue resistance, especially in those regions subjected to high operational stresses.